Telephony equipment originally designed to service 4-kHz voice-grade channels is still a major component of many telephone systems. Customer owned switches, the most common version being the Private Branch Exchanges (PBXs), are used to service a large portion of business communication requirements using voice grade systems. In a typical PBX installation, the PBX couples a plurality of extensions to the Public Switch Telephone Network (PSTN) eliminating the need to have an outside line for each extension. The customer owned switch routes calls between the connected extensions and the PSTN and may also provide extended functions such as call greeting, directory assistance, voice mail, call parking and paging functions. The facilities in which PBX's are used include hospitals, office complexes, corporate offices, factories, warehouses, sports arenas, schools, universities and other facilities having a relatively large number of telephone extensions but not requiring dedicated connections to each extension.
With the increasing reliance on data transmission such as information sent from and to computers and fax machines, voice communications are but one portion of the total communication requirement of many users. In the past, data was placed on voice equipment but newer systems are primarily concerned with transferring data, and voice transfer is a subsidiary function. Efficient ways to place voice from the PSTN on data networks, such as Local Area Networks (LANs), are needed.
LANs provide a privately owned, high bandwidth, transmission medium within a limited geographic area. LANs are designed to carry digital data at higher speeds than is possible using traditional telephone lines. A system of multiple LANs can be combined together to form a Wide Area Network (WAN). Many of the businesses and institutions that use PBXs to connect traditional telephones to the PSTN have also installed LANs.
Digital signals used on LANs are not compatible with the voice circuits of public telephone network. Many older PBX systems cannot transfer digital signals to the PSTN and thus computers at these locations must be directly connected to the PSTN by modulators/demodulators (modems) which commonly use frequency shift keying to communicate via the public telephone system. Even in newer PBX systems, even though the base technology of the PBX allows pass through rates of 64 Kbps, the terminal equipment poses fundamental limitations that limit the accessible bandwidth to a much lower rate. Often a dedicated line from the digital device to the PSTN is used, circumventing the PBX and increasing the costs of the communications system. None of the circuits mentioned provide the ability to adapt LAN voice, or Voice over IP, to the TDM switched environment of the PBX or PSTN. There is a need for a method and device which enables digital devices, such as personal computers, that communicate on a LAN to connect to the PSTN for the purpose of interworking voice traffic through an existing PBX system.
The prior art has attempted to solve the PBX/LAN interface problem by installing a card in a computer connected to both the LAN and the PBX that will allow digital communication with the PBX. See U.S. Pat. No. 5,636,218 to Ishikawa et al. and U.S. Pat. No. 5,649,005 to Lynch et al. as typical examples. The obvious shortcoming of this solution is that it requires the use of a computer to provide the conversion of the digital information and bandwidth is limited. Computer memory must be dedicated to performing a PBX compatible conversion instead of processing data. Computers are expensive and their capacities are poorly utilized in the pedestrian role of PBX/LAN traffic intermediary. Additionally, software must be available for the particular LAN protocol and PBX architecture the user wishes to connect. The present invention overcomes the deficiency in the prior art by adding a card to an existing telephone that can be connected to the LAN and acts as a bridging mechanism between the LAN and the PTSN/PBX.
Telephones are designed specifically to act as communication devices and as such are much more efficient bridge between the LAN and the PSTN than a computer and modem combination. The present invention relates to a system and method for transmitting data from a network through a telephone to the PSTN by using the inherent capabilities of the telephone system.